A bass loudspeaker, or woofer, radiates sound both in the forward and rearward directions. One of the purposes of a speaker enclosure is to prevent the cancellation effect of the rear wave of the woofer upon the waves radiated from the front by isolating the forward wave from the rearward wave. Several kinds of enclosures are known in the art:
a) Infinite Baffle (Air Suspension): An air suspension enclosure is a completely sealed box in which the rear prevented from cancelling the front wave. In addition, air trapped inside the sealed enclosure results in a springing effect which helps to stabilize the woofer cone. Since the resonant frequency of the enclosure determines the bass response, the smaller the enclosure, the higher the resulting resonant frequency. Air suspension designs are generally less efficient because the rear wave of the speaker is not being utilized to augment the low frequency output in any way.
b) Bass Reflex: The bass reflex design utilizes a portion of the rear wave of the woofer to augment the front wave. A precisely determined hole (vent) in the enclosure or a port tube or other such device is provided through which the rear wave from the woofer is permitted to exit the enclosure. The vent is designed to delay the rear wave so that when it emerges at the front of the enclosure, it is in phase with the front wave at the frequency the designer wishes to reinforce. As a result of such reinforcement, the bass reflex design offers increased efficiency. However, a disadvantage of this design is that the frequency response falls off very rapidly when the speaker system is called upon to reproduce frequencies below its system resonance.
c) Horn Enclosure: In this design, a horn acts as an acoustical transformer that matches the high mechanical impedance of the vibrating diaphragm to the relatively low acoustical impedance of the air at the large mouth of the horn. By virtue of this transformer action, the comparatively small area of the diaphragm finds it easy to "grab hold" of a large quantity of air by easy small steps through the gradually expanding cross sectional area of the horn through which the sound travels outward. Because of this effect, the horn provides more efficient coupling between the speaker enclosure and the air, and the efficiency of the horn enclosure is the highest of any known design. Horn enclosures may take many different shapes such as folded horns, corner horns, exponential horns, but the principles of operation are the same for all such shapes. A significant disadvantage of the horn design is that the length of the horn must be extremely long in order to obtain good bass response at low frequencies. Unless the horn is folded, this design is not practical in a compact speaker design.
d) Acoustical Labyrinth: This design channels the rear wave from the woofer through a folded passageway so that when the sound finally emerges it is delayed as much as possible and, therefore, reinforces the woofer at the lowest possible frequency. Like the horn enclosure, this design is not practical for compact speakers due to the length of the passage required.
In the design of commercially practical compact speaker enclosures, the two most viable designs are bass reflex and air suspension. In both of those designs, in order to obtain a deeper bass response, with all other factors equal, a larger enclosure volume is required. Enclosure volume is determined by the height, width and depth of the box. The current trend in loudspeaker design is towards smaller speaker enclosures because they intrude less in a room. Furthermore, a deeper enclosure is generally more compatible with aesthetic considerations than is a wider or taller enclosure. However, with a deep enclosure, the quality of the bass suffers. This is caused by the excessive depth of the enclosure. The sound wave which emanates from the back of the woofer travels to the rear wall of the enclosure and is reflected back towards the woofer. Because of the increased depth, when the wave returns to the woofer, it is out of phase with the woofer at a lower frequency where it causes cancellation and degrades the speaker's response. The result, in non-technical terms can be called "boomy" or "muddy." In a speaker enclosure that has a smaller depth dimension, there is less of a distance to travel. Therefore, cancellation occurs at a higher frequency. Usually this frequency of cancellation is above the sound range that the woofer is asked to reproduce and no degradation in performance is observed.
Numerous variations on the basic speaker designs discussed above have been tried in an effort to improve speaker performance. For example, U.S. Pat. No. 4,064,966 describes a loudspeaker system in which a pair of speakers are arranged in back-to-back relationship on opposite sides of a sealed enclosure. One speaker faces the listener while the opposite speaker faces into an acoustic labyrinth adjoining the rear of the sealed enclosure. The path-length of the labyrinth is such that the phase of the rear facing speaker's output is reversed, and therefore reinforces the output of the front facing speaker at a desired frequency. In this system, a pair of speakers are required to achieve the desired augmentation. Furthermore, the acoustic labyrinth comprises parallel walls that create standing waves. Such standing waves have the effect of increasing the output at certain unwanted frequencies and diminishing other frequencies and, in addition, tend to minimize the effective volume of the enclosure.
U.S. Pat. No. 2,205,804 describes a cabinet for a loudspeaker in which the speaker is disposed in a partition that divides the cabinet into two chambers. Inclined reflectors in each chamber direct sound from the front and rear of the speaker towards openings in the respective chambers. This effectively increases the length of the air path between the front and rear of the speaker, thus decreasing cancellation of the front and back waves at low frequencies. In this design, both the front and rear waves from the speaker are directed into the interior of the cabinet, thereby reducing the overall efficiency of the speaker system. Furthermore, the length of the air path between the front and rear of the speaker is a direct function of the height of the speaker cabinet, and thus a tall cabinet is required to improve low frequency response.
Folded horn enclosures are described in U.S. Pat. Nos. 2,751,997 and 4,853,964. A variation of this type of enclosure is described in U.S. Pat. No. 3,112,006. These speakers all attempt to provide an acoustic path for the rear wave having an exponentially increasing cross section.
U.S. Pat. No. 2,766,839 describes a vented enclosure for a loudspeaker. The top wall of the speaker enclosure incorporates a plurality of vents or ports. An inclined partition is provided inside the enclosure also having a plurality of ports. The purpose of this partition is to minimize the amplitude of the upper resonance peak that is inherent in a ported enclosure design. Other approaches to improving the bass response of a loudspeaker system are described in U.S. Pat. Nos. 3,186,509; 3,729,061; 3,912,866; 4,122,302 and 4,266,092.
The present invention represents an improvement over prior art loudspeaker systems such as those discussed above. One of the objects of the present invention is to provide a speaker system that yields a lower or deeper response from a given-sized enclosure over conventional methods of tuning. Another object of the present invention is the elimination of various unwanted side effects of conventional tuning methods. For example, the loudspeaker enclosure described herein avoids the introduction of parallel surfaces inside the speaker enclosure. As discussed above, parallel walls cause standing waves which degrade the speaker's performance.